Programmable Controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with the stored program. In Programmable Controllers such as those disclosed in U.S. Pat. Nos. 3,810,118; 3,942,158; 4,165,534; and 4,442,504 for example, the control program is stored in a memory that includes instructions which are read out in a rapid sequence and executed to examine the condition of selective sensing devices on the controlled equipment, or to energize or de-energize selected operating devices on the controlled equipment, contingent with the status of one or more of the examined sensing devices.
The sensing devices and operating devices on the controlled equipment are connected to the programmable controller through input and output circuits. Often the sensing devices are limit switches which are closed when a portion of the controlled equipment moves to a given position or when an article that is being manufactured passes by a point on the assembly line. The sensing device closes a switch allowing an electric current to flow back to the programmable controller. Typically the sensing device switches an alternating current having a frequency equal to the line current of the power supply to the programmable controller, which in the United States is 60 Hz. This limits the frequency at which the switch may be opened and closed to no greater than about one half of the line frequency or 30 closures per second in order for each closure to be detected. While this is adequate for most operations of the controlled equipment, some sensors produce output signals at a much higher frequency requiring a different type of input to the programmable controller.
One type of such sensing devices which produce output signals having a higher frequency are position encoders. The encoder has a shaft which is connected to the piece of equipment being controlled so that the shaft will rotate in response to the movement to be sensed. This movement may be rotational such as from the shaft of an electric motor or linear with a mechanism to convert it to rotational motion for sensing. Conventional encoders produce two pulsed output signals in response to the rotation of the shaft. The two signals are in quadrature so that the pulses of the two signals are 90 degrees out of phase. By determining which one of the two signals is leading the other, the direction of the shaft rotation may be determined; and the number of pulses per unit of time determines the speed at which the shaft is rotated. Typically, the frequency of the encoder signal is greater than 60 Hz rendering conventional programmable controller inputs incapable of handling the encoder's output.